CSCI-1200 Computer Science II — Fall 2007
Homework 6 — Sliding Block Recursion
In this assignment you will write a recursive solver for simple two-dimensional sliding block puzzles. A
sliding block puzzle consists of a frame and a set of rigid pieces that fit, but do not completely fill, the
space inside the frame. The goal of the puzzle is to transform the pieces from an initial configuration to an
end configuration by sliding the pieces within the free space of the frame. The pieces cannot be removed
or lifted or allowed to overlap in any way. For example, the puzzle below can be solved in 4 moves:
c
a b
a c
b a b
c
cb
a a
b
c
START GOAL
For this assignment, we will restrict our puzzles to rectilinear frames with integer dimensions, so they may
be represented with a 2D rectangular grid. Furthermore, the pieces will also have integer dimensions and
right angles, allowing them to be described and stored on a 2D grid. However, the pieces are not necessarily
rectangular; for example, we can have an ’L’ shaped piece. In solving the puzzle, the pieces will move one
piece at a time, one grid unit at a time, in one of the 2 axes (up, down, left, or right), and are not allowed
to rotate. Please read the entire handout before beginning your implementation.
There are many excellent websites dedicated to this deceptively simple genre of mechanical puzzles. We
encourage you to browse the web and play with some of the online versions of these puzzles. Some great
examples include:
http://www.johnrausch.com/SlidingBlockPuzzles/
http://www.puzzles.com/PuzzleLinks/SlidingBlockPuzzles.htm
But as usual, you are not allowed to search for, study, or use in any way anyone else’s code or pseudocode
to complete this assignment!
Your Tasks
For this assignment you will write a C++ program that uses recursion to solve simple sliding block puzzles.
Your program will read the starting and ending configurations from a text file that is specified on the
command line. An optional argument, the maximum move search depth, may also be specified on the
command line. Here are some sample command lines:
sliding_block.exe puzzle1.txt
sliding_block.exe puzzle2.txt -maximum_moves 10
The input file will contain the dimensions (width & height) of the puzzle frame, and then an ascii represen-
tation of the starting and ending configurations. Each piece will be identified with an alphabetic character.
Empty space will be represented with the ’_’ character. For example, the data file on the left corresponds
to the puzzle shown on the right:
4 3
bba_
baa_
____
_abb
aab_
____
?
ba
START
a
GOAL
b
